The development of a solid oxide fuel cell, having a layered structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer (oxidant electrode layer) and a fuel electrode layer, is progressing as a third-generation fuel cell for use in electric power generation. In a solid oxide fuel cell, oxygen (air) is supplied to the air electrode section and a fuel gas (H2, CO and the like) is supplied to the fuel electrode section. The air electrode and the fuel electrode are both made to be porous so that the gases can reach the interfaces in contact with the solid electrolyte.
The oxygen supplied to the air electrode section passes through the pores in the air electrode layer and reaches the neighborhood of the interface in contact with the solid electrolyte layer, and in that portion, the oxygen receives electrons from the air electrode to be ionized into oxide ions (02−). The generated oxide ions move in the solid electrolyte layer by diffusion toward the fuel electrode. The oxide ions having reached the neighborhood of the interface in contact with the fuel electrode react with the fuel gas in that portion to produce reaction products (H2O, CO2 and the like), and release electrons to the fuel electrode.
The electrode reaction when hydrogen is used as fuel is as follows:
Air electrode: (½)O2+2e−→O2−
Fuel electrode: H2+O2−→H2O+2e−
Overall: H2+(½)O2→H2O
FIG. 5 shows the internal structure of the electric power generation cell 1 in a conventional solid oxide fuel cell; in this figure, reference numeral 2 denotes an air electrode (cathode) layer, reference numeral 3 denotes a solid electrolyte layer, reference numeral 4 denotes a fuel electrode (anode) layer, and the air electrode layer 2 and the fuel electrode layer 4 are arranged respectively on both surfaces of the solid electrolyte layer 3 so as to sandwich the solid electrolyte layer 3 therebetween. Conventionally, ingeneral, a power cell has a structure in which each layer is formed as a single layer as shown in the figure.
Here, the air electrode layer 2 and the fuel electrode layer 4 need to be formed of materials having high electronic conductivity. Because the air electrode material is required to be chemically stable in the oxidative atmosphere of high temperatures around 700° C., metals are unsuitable for the air electrode, and generally used are perovskite type oxide materials having electronic conductivity, specifically LaMnO3 or LaCoO3, or the solid solutions in which part of the La component in these materials is replaced with Sr, Ca and the like. Additionally, as the fuel electrode material, generally used is a metal such as Ni or Co, or a cermet such as Ni-YSZ or Co-YSZ.
Because the solid electrolyte layer 3 is the medium for migration of the oxide ions and also functions as a partition wall for preventing the direct contact of the fuel gas with air, the solid electrolyte layer 3 has adense structure capable of blocking gas permeation. It is required that the solid electrolyte layer 3 is formed of a material having high oxide ion conductivity, and chemically stable and strong against thermal shock under the conditions involving the oxidative atmosphere in the air electrode layer section and the reductive atmosphere in the fuel electrode layer section. As a material which can meet such requirements, generally used is an yttria stabilized zirconia (YSZ) which displays a relatively high oxide ion conductivity at high temperatures; however, in these years, the operation temperature of solid oxide fuel cells tend to be lowered, and accordingly ceria based oxide materials (ceria added with samarium) have come to be used which are slightly weak in the high temperature reductive atmosphere, but display excellent electric conductivity at low temperatures. Additionally, Japanese Patent Laid-Open No. 2001-52722 discloses solid oxide fuel cells in which as the solid electrolyte, used are the lanthanum gallate based oxide materials displaying high oxide ion conductivities.
As described above, there have hitherto been made many researches and improvements on the materials and the like for the solid electrolyte layer; additionally, various improvements have hitherto been made on the fuel electrode layer.
A power cell in which an yttria stabilized zirconia or a samaria doped ceria (SDC) is used as the solid electrolyte layer 3 has a drawback such that the internal resistance in the air electrode section becomes large; in particular, in the case of the samarium loaded ceria, the samarium loaded ceria displays an excellent electric property at low temperatures, as described above, but is a mixed electron-oxide ion conductor, and is low in the proportion of the oxide ion conductivity, which is a cause to raise the internal resistance. Additionally, a power cell in which a lanthanum gallate based oxide material is used as the solid electrolyte layer has a tendency such that on the contrary to the above described case, the internal resistance in the fuel electrode section becomes large, and additionally has a drawback such that the lanthanum gallate based oxide material is relatively expensive. Anyway, when the internal resistance is high, the IR loss is increased and no efficient electric power generation can be expected.
On the other hand, in a power cell provided with a fuel electrode layer 4 having a single layer structure as shown in FIG. 5, an excellent electric power generation property (current-voltage-electric power property) is displayed for the short term test of the electric power generation, but a problem involving the durability has been revealed to remain unsolved in the long term test of the electric power generation.
The electric power generation cell in a solid oxide fuel cell is required to have a durability of 40 to 50 thousands hours for practical use; in the case of the electric power generation cell of a conventional structure shown in FIG. 5, a degradation of the electric power generation property is found in a durability test for about 100 hours. The conceivable main causes for the degradation include the phenomenon of the exfoliation between the solid electrolyte layer 3 and the electrode layers (particularly the fuel electrode layer 4) and the mutual diffusion of the metal elements between the solid electrolyte layer 3 and the electrode layers.
As for the exfoliation of the fuel electrode layer 4, it is conceivable that the metals such as Ni contained in the fuel electrode layer 4 are baked to the solid electrolyte layer 3 in the conditions of oxides, and the fuel electrode layer 4 is exfoliated from the solid electrolyte layer 3 owing to the sintering shrinkage caused by the reduction at the time of electric power generation; additionally, it is also conceivable that the diffusion of the Ni and the like, which are the materials for the fuel electrode layer 4, into the solid electrolyte layer 3 degrades the performance of the solid electrolyte layer 3.